Eruptions of the Yellowstone volcanic system have included the two largest volcanic eruptions in North America in the past few million years; the third largest was at Long Valley in California and produced the Bishop ash bed.
On the diesel Passat, the TDI badge stands for Turbocharged Direct Injection, which isn't really all that helpful as the gasoline-powered Passat is also turbocharged and also uses direct injection. Volkswagen attempts to clear this up by calling this Passat the Passat TDI Clean Diesel.
The highest-volume model is the Volkswagen Jetta TDI, but VW also offered TDI versions of the Passat, the Golf, the Jetta SportWagen, and the Beetle. The recall will also apply to the Audi A3 TDI, in two generations: both the one sold from 2009 through 2013, and the new version introduced for 2015. But across those models there are actually two engines in three versions. The most common is the 140-hp EA188 2.0-liter turbodiesel four introduced for 2009.